This invention relates to web pressing. More particularly, the invention relates to the presing of a traveling web, such as paper, freshly formed from an aqueous slurry of fibers in a press which includes a non-rotating press member, such as a shoe. Still more particularly, this invention relates to a press of the type known in the papermaking industry as an extended-nip press wherein the web of paper travels through an area of contact between a moving surface, such as a rotating roll, and a felt and belt carried on the roll surface and pressed against it by a pressurized shoe having a face with a contour matching an arcuate portion of the roll surface.
While such extended-nip type presses are relatively new in the papermaking industry, there have been numerous installations and various types of configurations. Common to all extended nip presses is the use of one or more looped, flexible belts which travel through the nip with the paper web and one or more felts. One side of the belt contacts the web or felt, and its other side is in sliding contact with a stationary element, commonly referred to as a shoe, which provides a surface area of contact.
Since all machinery is not perfectly aligned, and in view of the inevitable deflection, however slight, across the span of a papermaking machine, and other small variations in the tolerances, construction or operation of the apparatus, the belt tends to migrate to one end of the press or the other during operation. In the past, this undesirable operating characteristic has been corrected by various means, such as by using movable guide rolls within the looped belt. Another manner of guiding the belt has been to mount the edges of the belt over a rotatable disc disposed on either end of the press which maintains the edges of the belt in a desired position relative to the extended nip during operation. Finally, guide plates have been utilized within the looped belt to maintain its location relative to the nip.
However, each of these prior methods of maintaining the belt in a desired position during operation of the extended-nip press has serious deficiencies. For example, guide rolls and guide plates disposed within the belt require additional equipment and produce additional friction, or drag, in the belt while performing their function of maintaining belt guidance.
The belt guide incorporating a rotating disc at either side of the belt perhaps provides the most positive belt guidance, but also produces the most serious deficiency because the discs, in order to operate effectively, must be biased axially outwardly, such as with springs, so the belt is under a tension in the cross machine direction. This stresses the belt and greatly increases its rate of wear since the belt is flexed through the nip in the direction toward the longitudinal axis extending across the machine while it is simultaneously being biased outwardly by the rotating discs.